Interpretive Summary: Some plant-infecting fungi secrete certain enzymes while infecting their plant hosts. One of these enzymes is called pectin methylesterase and it has been found to be the key in causing disease by a number of fungal species on a variety of plant species. We studied an important pepper pathogen called Phytophthora capsici, and identified nine specific genes in the pathogen for producing the pectin methylesterase enzyme. We studied how the enzyme was produced in allowing the fungus to infect three different types of plant: pepper, tomato, and cucumber. We found that production of the enzyme was more critical for disease expression during the later stages of infection compared with early stages of infection. Thus, the later stages seem to be the most critical time for the fungus to secrete this enzyme. The results help us to understand how enzymes can be involved in causing disease on hosts such as pepper, tomato, and cucumber by the pathogen Phytophthora capsici.

Technical Abstract:
Phytophthora capsici causes damage on many plant species, and secretes various pectin methylesterases during all stages of infection. We identified nine Pme genes (Pcpme 1-9) from a genomic library of highly virulent P. capsici strain SD33 and further analyzed the expression pattern of nine genes on three hosts: pepper, tomato, and cucumber using qRT-PCR during all stages of infection. All nine genes were found to be differentially expressed in three host species in the course of P. capsici interaction. The expression levels of the respective genes increased from 1 to 7 dpi in pepper, while most genes presented a decreasing trend of expression from 1 to 5 dpi in tomato fruits. However, in both fruits peaks of expression were reached at 7 dpi. In cucumber fruits, each gene showed minor expression levels from 1 to 3 dpi, exhibited definite peaks at 5 dpi, and then decreased from 5 to 7 dpi. Thus, evidence from our studies of Pcpme gene expression in different plants at a range of time points suggests that the late stages of infection may represent the most critical time for P. capsici to successfully express or/and secret PMEs.